One of the most common fuel combustion power generation systems is the closed Rankine cycle, employed in various forms is steam power plants around the world. In the closed Rankine cycle, a working fluid, most often water, is first pumped to a high pressure. The water is then heated in a boiler, where the water boils into high temperature high pressure steam. This steam is then expanded, typically in multiple stages of turbines. The turbines output power from the system. The steam is discharged from the turbine, and enters a condenser, where it is cooled back into a liquid and delivered back to the pump to repeat the cycle.
Heretofore, fuel combustion based steam power plants have emitted pollution due to the products of combustion of the fuel being too difficult to capture or neutralize entirely, in an economic fashion, and so requiring release. Theorists have proposed that emission of pollutants could be reduced or eliminated by combusting the fuel with oxygen, rather than air (including nitrogen), and then capturing the gaseous combustion products (primarily CO2). Recently one company, Clean Energy Systems of Rancho Cordova, Calif., has demonstrated such a steam power plant.
One common attribute of such proposed and demonstrated power plants is that the working fluid is not externally heated as with a conventional closed Rankine cycle power plant. Rather, the working fluid driving the turbine or other expander has been the actual products of combustion, with possibly water or CO2 added to the products of combustion. In gas turbine power plants operating on the Brayton cycle, such “internal” rather than “external” heating has been effective, partly due to the Brayton cycle power plants typically being configured as open cycles rather than closed cycles.
One consideration in the configuration of a power plant is the minimization of wear on the equipment, so that maintenance can be minimized and useful life of the equipment can be maximized. One source of equipment wear is corrosion. In a conventional steam power plant, the working fluid is typically carefully filtered pure deionized water. Such water exhibits very low corrosive effects on the boiler, turbine, condenser and other parts of the steam power plant. It is desirable to maintain such a low corrosivity advantage when designing or converting a Rankine cycle power plant to operate with zero emissions, as explained above. However, the working fluid of such prior art zero emissions power plants has been a combination of CO2 and water. Such a combination readily forms carbonic acid, having a moderate level of corrosivity with most commonly employed equipment materials. Also, because the heating is “internal,” any other pollutants in the products of combustion pass through the turbine, and other equipment in the plant, enhancing the potential for increased maintenance and decreased useful life.
Accordingly, a need exists for a zero emissions power generation system which employs the closed Rankine cycle, and yet maintains the benefits associated with external heating and keeping the working fluid pure water.